Events which occur at enzyme active sites can lead to intramolecular shifts in protein structure and subsequent modification of external cellular function. In the research proposed here we intend to investigate the molecular bases for this phenomenon in two proteins, mitochondrial cytochrome oxidase and the photoregulatory protein, phytochrome. The former enzyme is the terminal mitochondrial oxidase and catalyzes the four electron reduction of dioxygen to water. It is responsible for 905 of the body's consumption of oxygen. It is also the locus of site III respiratory control and recent work has shown that it contributes directly to the transmembrane proton gradient by pumping protons stoichiometrically with electrons transported. Resonance Raman, electron paramagnetic resonance and rapid-scan, stopped flow kinetic studies of the enzyme and appropriate model compounds are planned in order to probe the relationship between active site structure, substrate redox state, and protein conformational and allosteric state. We also intend to extend our model compound work in order to assess the symmetry properties of the enzyme heme a chromophore. Plant phytochrome, an important encyme in growth and development processes, can be viewed as well as a model for other photoreceptor proteins. We plan a resonance Raman approach to elucidate the molecular structures of the active resting forms of this protein as well as of the intermediate states involved in the p]hototransformation between these two forms. A knowledge of these structures and, in particular, the bonding between protein and chromophore, is expected to provide insight into the mechanism by which photon absorption and chromophore rearrangement are ultimately expressed at the cellular level.